Known in the art is a process for preparing fluoroaromatic compounds by reacting a chloroaromatic compound with potassium fluoride in an aprotic polar solvent (sulfolane) at a temperature of 210.degree. C., as a catalyst use being made of isoalkylpyridinium salts of the formula ##STR1##
wherein R.sup.1 and R.sup.2 are monovalent or diva ent radicals selected independently from substituted on unsubstituted hydrocarbon radicals having from 1 to 13 carbon atoms or divalent alkylene radicals capable of forming a cycle containing from 4 to 8 carbon atoms, R.sup.3 is a residue of the formula CH.sub.2 C(R.sup.5)HR.sup.4, R.sup.4 and R.sup.5 =alkyl C.sub.1 -C.sub.8, X=F, Cl, Br (U.S. Pat. No. 4,927,980, C07C 79/12, publ. May 22, 1990). The preferred catalyst is N-(2-ethylhexyl)-4-(N',N'-dimethylamino)pyridinium chloride.
In the description of the process no data can be found on the possibility of preparing highly fluorinated compounds, such as hexafluorobenzene and pentaflorobenzene; examples are presented, illustrating the substitution of only one halogen atom in the aromatic compound. From 4-chloronitrobenzene a mixture is obtained which comprises 48% of 4-fluoronitrobenzene and 45% of 4-chloronitrobenzene. Conversion of 4-chloronitrobenzene is 45%.
The process is disadvantageous in a low yield of target products and using a solvent which must be separated from the reaction products and regenerated.
A process is known for preparing polyfluoroaromatic compounds of the formula C.sub.6 Cl.sub.n Br.sub.m F.sub.p (n and m=0-6, p=0-5, n+m+p=6) by reacting a haloaromatic compound with an alkali metal fluoride in the presence of an aminophosphonium catalyst, preferably tetrakis(dialkylamino)phosphonium halides, which is used in an amount of 3-6 mol. % of the molar quantity of the starting haloaromatic compound (U.S. Pat. No. 5,624,827 C07C 25/13, publ. Oct. 20, 1998). The process is carried out in a reactor at a pressure of 5-7 kg/cm.sup.2 without a catalyst at a temperature of 150-350.degree. C. The total molar yield of hexafluorobenzene and pentafluorobenzene is 70-86% with the hexachlorobenzene conversion of 80-95%.
It is also proposed to carry out the reaction in the liquid phase, for instance, in an aprotic polar solvent or in haloaromatic compounds--dichlorotetrafluorobenzene, trifluorotrichlorobenzene and others, which are in the liquid state at the reaction temperature. In these particular embodiments of the invention with the use of solvents particular examples of preparing fluoroaromatic compounds are not presented.
The process is disadvantageous in the use of a reactor under a pressure, in obtaining a mixture of polyfluorobenzenes instead of one target product, as well as in using catalysts whose synthesis is technologically labor-intensive.
The most relevant technical solution is a process for preparing hexafluoro- and pentafluorochlorobenzenes, which consists in treating hexafluorobenzene with potassium fluoride in sulfolane in the presence of a catalyst, 18-crown-6-macrocyclic polyether. 0.59 mole of the catalyst is used per mole of hexafluorobenzene. The reaction is carried out at a temperature of 180-200.degree. C. for 24 hours. The resulting mixture contains 23-24% of C.sub.6 F.sub.6, 44-45% of C.sub.6 F.sub.5 Cl and 30-31% of C.sub.6 F.sub.4 Cl.sub.2. The total yield of hexafluorobenzene and chloropentafluorobenzene as calculated for the charged hexafluorobenzene is 45% (RF Inventor's Certificate No. 676864, C07C.sub.25/13, publ. Apr. 30, 1994).
The process is disadvantageous in the use of a costly catalyst, in the process being periodic, and in obtaining a mixture of polyfluorobenzenes with a low yield of target products.